The present invention is concerned with the oxidation of sulfur in weak black liquor and with the elimination of odorous sulfur emissions and the production of sulfate soaps in kraft pulp mills.
In the manufacture of paper by the kraft or sulfate process, sulfur is present in the chemical solution used to cook wood to obtain fiber, as sodium sulfide, or other sulfur compounds or elemental sulfur. The use of sulfur enhances the separation of cellulose fiber from wood and has other benefits, including stronger fibers, fewer rejects of incompletely separated material, and less dirt.
Unfortunately, the use of sulfur results in the formation of odorous chemical compounds such as hydrogen sulfide, mercaptans, disulfides, and the like, which are released at many points in the kraft pulping operation. Odorous compounds are emitted in small quantities at the digester, through gas-off of non-condensables during the cooking reaction. At the end of the digestion reaction when the digester is emptied to a blow tank, odorous chemicals are released. In the washing-filtration operation for the separation of dissolved non-cellulose parts of the wood and cooking chemicals from the fiber, odorous chemicals are stripped to the atmosphere. The disposal of woody material removed in the washing operation and the recovery and storage of cooking chemical solutions also produce odorous emissions. In the concentration of the stored solution by an evaporation system, odorous sulfur compounds are again emitted. Following concentration, when the solution of wood-chemical material is burned in a furnace to produce steam and molten smelt from which the cooking chemicals are recovered, odorous emissions are again released.
The paper industry has attempted to eliminate or control the creation and emission of odorous chemicals for a long time. In an effort to solve the problem, oxidation of sulfur and sulfur compounds in black liquor has been extensively investigated. It has been proposed, for example, to add oxygen to the digester, but possible adverse effects upon pulp quality have been noted in some instances, and prohibitively high oxygen requirements have been encountered at the elevated temperatures and pressures in the digester. It has also been proposed to oxidize weak black liquor (before concentration), but large capital investments have been required, and the practice has been discouraged in mills using coniferous woods because of the production of high volumes of stable foam whenever weak resinous black liquor is sparged with air. When the resinous weak black liquor is concentrated and most of the sulfate soaps are skimmed off, the strong black liquor exhibits a much reduced foaming propensity, and it is current practice to oxidize the black liquor after concentration, with air. However, weak black liquor oxidation has certain advantages over concentrated black liquor oxidation, including reduction of corrosion in the evaporators and improved heat transfer, reduction of odorous non-condensable gases in the evaporator condensate, recovery of the sulfur values through the stabilization of the odorous non-condensable gases, and lower horsepower requirement. Moreover, strong black liquor oxidation fails to treat the problem of odorous emissions from many of the sources in the kraft process.
As alluded to above, sulfate soap, a byproduct of the manufacture of pulp, particularly from coniferous wood, is a major factor in the creation of foam when air, with its high nitrogen content, or other inert gas is bubbled through black liquor. The black liquor from highly resinous woods produces a foam that is most difficult to break. Nevertheless, sulfate soap is a highly desirable byproduct, because it may be reacted with sulfuric acid to form tall oil, a useful article of commerce. At present, only part of the sulfate soap is extracted from the black liquor.
In general, the prior art suffers from one or more of the following deficiencies: inadequate fixation of sulfur, stripping of sulfur to the atmosphere by air used in oxidation, excessive and/or uncontrolled use of oxygen (with resultant oxidation of organics, reduction of heat value of black liquor, reduction in tall oil quality, and explosions), inability to treat black liquor containing significant fiber concentrations, degradation of pulp quality, foaming, difficult sulfate soap separation, poor sulfate soap yields and quality, complexity, impracticality, unreliability, frequent maintenance problems, and high cost.